The invention relates to a scanning device having a stationary section comprising a frame and movable section comprising a lens, which movable section is movable along a first axis and a second axis oriented transversely of said first axis, which device comprises a first actuating means and a second actuating means adapted to cooperate magnetically with said first actuating means v/a an air gap, one of said first and second actuating means comprising at least one focusing coil for moving the lens along the first axis and further comprising two pairs of tracking coils, which pairs extend parallel to one another, for moving the lens, which is disposed between the two pairs, along the second axis, which tracking coils of each pair are disposed along a line parallel to the second axis and have coil axes oriented transversely of a plane defined by the first axis and the second axis, the other means of said first and second means comprising magnets.
A prior art scanning device is known from EP-A 0,178,077 (herewith incorporated by reference). The known scanning device for scanning an optical recording medium by means of a scanning spot comprises a stationary base and a body which is movable relative to this stationary base. The movable body carries a lens and two magnets fixed to side surfaces of the movable body which face away from one another. Opposite the magnets, the base carries focusing coils for moving the movable body in two opposite focusing directions and tracking coils for moving said body in two opposite tracking directions transverse to said focusing directions, one pair of tracking coils being arranged opposite each magnet. The magnets have a coincident magnetic axis oriented transversely of the focusing directions and the tracking directions. The tracking coils situated opposite the magnets have vertical active coil portions, i.e. coil portions which extend in the focusing directions, which coil portions are situated in an area directly opposite a magnet pole in a center position of the movable body. Vertical inactive coil portions are situated outside this area and are connected to the active coil portions by horizontal inactive coil portions. The focusing coils have horizontal active coil portions for moving the lens into focus positions.
In the prior-art electro-optical scanning device, which is used in a focus and tracking control system, stray fields of the magnets result in undesired magnetic forces being exerted on the movable section by the horizontal inactive coil portions when the tracking coils are energized. In the center position, particularly viewed in the focusing direction, these forces on a tracking coil cancel one another because of the symmetry in the assembly of magnets and tracking coils. However, if the lens is in a focus position, in which the movable body has been moved in an upward or downward direction relative to the center position, oppositely directed magnetic forces of different magnitude are produced at the mutually parallel horizontal inactive coil portions of each tracking coil. In a pair of tracking coils this gives rise to two parallel opposite disturbing forces, as a result of which the movable body is subjected to undesired torques about an axis transverse to the focusing direction and the tracking directions. During tracking this may result in substantial resonances of the movable body about said axis, so that the scanning spot performs undesirable to and fro excursions, which are interpreted as additional error signals by the control system. In the case of a large amplitude of the undesired excursions these error signals may give rise to complete failure of the control system.
The tracking coils of the prior-art scanning device have active vertical coil portions which are long relative to the height dimensions of the magnets. This has the drawback that it leads to a large overall height. Another drawback is that the tracking coils are used inefficiently because the magnetic forces to be produced in a tracking direction are small in comparison with the required energy as a result of unnecessary heat losses. Both drawbacks can be mitigated by the use of shorter active vertical coil portions. However, a drawback of applying such a measure to the prior-art scanning device is that when the tracking coils are energized undesired torques about the axis transverse to the focusing directions and tracking directions may be generated which amplify the undesired torques already mentioned above. The undesired torques, as result of the use of short active vertical coil portions, arise in focus positions of the lens in which the magnets have been moved in a focusing direction relative to the tracking coils because the points where the magnetic actuating forces act on the movable body are shifted in a focusing direction when the tracking coils are energized.